1. Field of Invention
The present disclosure relates to downhole electric submersible pump (ESP) systems that are submersible in wellbore fluids. More specifically, the present disclosure involves a method for controlling the loading applied to the radial bearings in an ESP to control the dynamic characteristics of the bearings in operation.
2. Description of Prior Art
Submersible pumping systems are often used in hydrocarbon producing wells for pumping fluids from within the wellbore to the surface. These fluids are generally liquids and include produced liquid hydrocarbon as well as water. One type of system used employs an electrical submersible pump (ESP). ESPs are typically disposed at the end of a length of production tubing and have an electrically powered motor. Often, electrical power may be supplied to the pump motor via a cable. The pumping unit is usually disposed within the well bore just above where perforations are made into a hydrocarbon producing zone. This placement thereby allows the produced fluids to flow past the outer surface of the pumping motor and provide a cooling effect.
With reference now to FIG. 1, shown in a partial sectional view is a cased wellbore 8 having an ESP system 10 disposed therein. The ESP system 10 is made up of a motor 12, a seal 14, and a pump 16 and is disposed within the wellbore 8 on production tubing 18. Energizing the motor 12 drives a shaft coupled between the motor 12 and the pump section 16. The source of the fluid drawn into the pump comprises perforations 20 formed through the casing of the wellbore 10; the fluid is represented by arrows extending from the perforations 20 to the pump inlet. The perforations 20 extend into a surrounding hydrocarbon producing formation 22. Thus the fluid flows from the formation 22, past the motor 12 on its way to the inlets.
Traditionally, ESP systems 10 include bearing assemblies along the shafts in the motor section, seal section, and pump. Often, the bearings are plain sleeve bearings that provide radial support. One example of a bearing assembly provided in a motor section is provided in a cross sectional view in FIG. 2. Shown is a shaft 24 with an outer sleeve 26 that is circumscribed by a stator stack 28. The sleeve 26 couples to the shaft 24, such as by a key, and rotates along with the shaft 24. A housing 30 encases the outer circumference of the stator stack 28. A bearing assembly 32 is set between the outer sleeve 26 and stator stack 28 that radially encompasses a portion of the sleeve 26. The motor bearing assembly 32 may have an insert 34 mounted on the outer circumference of the sleeve 26; a bearing carrier 36 encircles the insert 34 and in the absence of an insert directly mounts on the shaft sleeve. A T-ring 38 may be included that mounts to the inner surface of the stator stack 28 for preventing bearing rotation. The sleeve 26, and therefore the shaft 24, is radially supported by the insert 34 or the bearing carrier 36. A lubricant film (not shown) allows for sleeve 26 rotation within the insert 34 or the bearing carrier 36.
Referring to FIG. 3, shown in a side sectional view is a prior art example of bearings in a pump section of an ESP system. Diffusers 40 are typically coaxially stacked in close contact within a housing 30. An impeller 42 is stacked between each successive diffuser 40, where each impeller 42 is coupled to and rotates with the shaft 24. Passages 44 curve radially and lengthwise throughout the diffusers 40 that register with passages 46 that similarly curve radially and lengthwise through the impellers 42. Rotating the shaft 24, and thus the impellers 42, forces fluid through the passages 44, 46 to pressurize the fluid as it passes along the stack of diffusers 40 and impellers 42. A sleeve bearing 48 couples around the shaft 24 to provide a bearing surface between the shaft 24 and inner circumference of the diffusers 40. As the shaft 24 rotates, a film of lubricating fluid is maintained between the bearing 48 and diffuser 40.